1. Field of the Invention
The present invention concerns a cyclone of the type having a classifier for at least partially separating particles according to size.
2. Description of the Prior Art
Conventionally, the first stage of dust collection from blast furnace waste gas is a dustcatcher. This is no more than a large vessel with low gas velocities in which coarse dust particles are allowed to settle out. The second stage is a wet scrubber where small particles are removed. Because of its composition, the dust captured in the dustcatcher can be recycled back to the blast furnace. Dust captured in the wet system must be disposed of in other ways because it contains materials such as zinc that cannot be recycled.
Dustcatchers invariably do not achieve an ideal split and much recyclable material is passed to the wet system along with the contaminants. A higher efficiency dust removal system is required that maximizes the recycle of good material whilst passing on the contaminants to the wet system.
A conventional dry dust collector is the cyclone. Unfortunately, the efficiency of a cyclone tends to be high enough to collect too much of the zinc bearing material.
Designing a cyclone to achieve a reduced efficiency is not straightforward. Often the dirty gas inlet conditions are not known accurately or are likely to vary during operation. The necessary efficiency might be unknown and is likely to vary depending upon changes in dust particle size distribution. During test work it has been found that varying the geometry of the cyclone does not always produce expected changes in dust collection efficiency. The efficiency of a cyclone may be changed at the design stage by reducing the inlet velocity. The effect of this would be to increase the size of the cyclone which consequently increases costs. The result would be a cyclone whose performance remained subject to the vagaries of inlet gas conditions and dust loading and size analysis.
The dirty gas from a blast furnace is conventionally delivered to the first stage cleaning plant via a duct known as a downcomer that slopes steeply, often at an angle between 40 and 55 degrees depending upon site layout. The entry to the cyclone is in the horizontal plane and is rectangular in section. To turn the gas flow into the horizontal plane the designer might consider the use of internal guide vanes, typically in the rectangular section, to improve the flow distribution entering the cyclone.